Circuit arrangements with interlinked or interconnected feedback control circuits frequently have a problem on start-up in that, because they are interdependent, the feedback control circuits block each other. In such cases the circuit arrangement then does not reach a stable operating point.
Published German application for patent No. 10 2004 030 812 A1, for example, describes a multibit delta-sigma converter that in addition to the usual outer feedback control circuit as a sigma-delta modulator also has a second inner feedback control circuit. The inner feedback control circuit consists of a quantizer as a tracking analog-digital converter.
FIG. 1 shows a corresponding sigma-delta converter. The sigma-delta converter converts an analog input signal ZA to a digital output signal ZD. To do this, a feedback signal ZF, that is subtracted from the analog input signal ZA by means of an adder, is derived from the output signal ZD by means of a device for dynamic element compensation DEM and a feedback digital-analog converter FDAC. The signal Z1 obtained in this way is applied to a filter CTF that is usually in the form of an integrator. By quantizing the filtered signal Z2 that is sampled at a clock frequency of a clock signal CLK, a digital signal ZD is obtained.
The quantizing is achieved according to published German application for patent No. 10 2004 030 812 A1 by means of a tracking analog-digital converter, that in its simplest form consists merely of a comparator, that compares the signal Z2 with an internal reference voltage ZR that is obtained from the output signal ZD by means of a digital-analog converter DAC. A counting device UDC downstream of the quantizer or comparator Q increments or decrements depending on the results of the comparison or the quantized signal Z3. The output voltage ZR of the digital-analog converter DAC also changes due to the change in this counter state.
Therefore, an outer feedback control circuit RK1 and an inner feedback control circuit RK2 are provided. Particularly after the sigma-delta converter is switched on, the difference between the two inputs of the comparator or quantizer Q is very large during the starting phase and shows as a quantizing error in the output signal ZD. On each clock pulse the counter UDC follows the value of the input signal ZA or Z2. Therefore, in the starting phase the output signal ZR of the digital-analog converter DAC approaches the signal Z2 and the quantizing error becomes smaller.
Since shortly after the sigma-delta converter is switched on the capacitors of the loop filter CTF are initially discharged and must then be recharged and further components must be brought to an operating state, it is possible that a stable operating point will not be found. Since the use of the tracking analog-digital converter means that the succeeding quantizer result depends on the preceding one, the quantizing or residual error is particularly large at the start of operation. Due to the mismatch in the coupling between the inner and outer control loops RK1, RK2, the presence of a large quantizing error in the inner loop can cause interference in the complete feedback control system RK1 so that the corresponding sigma-delta converter never finds a stable operating point.
It was therefore proposed that the tracking quantizer be operated during starting as a (reduced bit width) flash digital-analog converter, i.e. to interrupt the inner feedback loop. Immediately the circuit has reached a stable operating point, a switch to tracking mode takes place at the instant of a zero crossing of the input signal ZA. It is, of course, difficult to detect the most satisfactory switching instant for a zero-crossing signal. Especially with multitone signals, that frequently require a sigma-delta converter, this is often impossible with fast-changing signal shapes.
The problem of an unstable behavior during the starting phase can also arise if instead of a tracking quantizer an analog-digital converter using the method of successive approximation with a noise shaper or internal sigma-delta converter is used. Generally the difficulties with stability in a starting phase frequently occur where several feedback control circuits are interlinked.